VATICAN CITY -- Retired Pope Benedict XVI has denied speculation that he was pressured to leave office, saying his decision was freely made and his alone.

Benedict wrote to the Vatican correspondent for La Stampa newspaper, Andrea Tornielli, amid a new round of speculation about his reasons ahead of Friday's first anniversary of the first resignation by a pope in 600 years.

"There isn't the slightest doubt about the validity of my resignation from the Petrine ministry," La Stampa quoted Benedict as writing. "The only condition for the validity is the full freedom of the decision. Speculation about its invalidity is simply absurd."

Benedict, 86, also defended his decision to continue wearing the white cassock of the papacy, saying that there were no other clothes available.

Media are again speculating about what drove Benedict from office. Italian journalist Antonio Socci suggested last week in the conservative daily Libero that the resignation may have been invalid, claiming Benedict was pressured by a group of cardinals opposed to him.

Benedict, however, had made clear from the start of his papacy that it might not end in his death. In the 2010 book "Light of the World," Benedict wrote: "If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign."

Benedict made a surprise cameo at this weekend's ceremony to formally install 19 new cardinals. It was the first time he and Pope Francis had appeared together at a public liturgical ceremony and immediately raised questions about whether Benedict might occasionally be reintegrated back into the life of the church.

British Cardinal Vincent Nichols, one of the new cardinals, said Benedict's presence didn't represent a change.

"He says his vocation now is to pray for the church and to live that life of prayerful seclusion, and he would appear to be very peaceful doing that," Nichols told reporters.

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Emeritus Pope Benedict XVI is shown in this file photo greeting the crowd from the central balcony of St. Peter's Basilica moments after being elected, at the Vatican, April 19, 2005 (AP / Domenico Stinellis)